This invention relates to a marine propulsion system and, in particular, to apparatus for providing the operator of a boat with certain continuous data regarding the drag on the hull of the boat as it is being propelled through the water by a transom mounted propulsion unit.
In U.S. Pat. No. 4,605,375 to Ferguson, there is described a typical stern mounted propulsion unit for a boat having a trim feature for adjusting the position of the propulsion unit to provide for more effective propeller thrust angles for existing running condition Running conditions can be effected by many things such as distribution of the boat load, water conditions and the angle of attack that the boat hull presents to the water, power settings and propeller pitch
The Ferguson unit contains a pair of hydraulically operated cylinders that are used to move the propulsion unit through a full range of trim positions. In addition, to the trim positions the cylinders can move the propulsion unit beyond the full-up trim position into a tilt position to protect the propeller from rocks and other objects when the boat is in shallow water or being trailered. Movement of the cylinders is controlled by means of an electrical system which regulates the operation of a pump for delivering hydraulic fluid to the cylinders. A pressure actuated switch is contained in the hydraulic system that is adapted to change status when the pressure in the fluids exceed a predetermined level. The cylinders are mounted on the boat so that changes in thrust will produce corresponding changes in the hydraulic fluid pressure. The pressure sensitive switch is set so that it will change status when the thrust exceeds a certain limit. This in turn prevents the propulsion unit from being moved from the trim configuration into the tilt configuration when the propeller is turning at high speed.
In most boat trim systems having one or more hydraulic cylinders for selectively adjusting the angular position of a propulsion unit, the boat operator has no way of judging when the boat is properly trimmed for the existing running conditions. Boat operators have heretofore been forced to rely upon their own instincts when trimming or adjusting the condition of the boat. Only the most experienced and skilled operators, however, can judge with any great accuracy when the boat is properly adjusted for existing running conditions. An improperly trimmed boat operating under adverse running conditions, can be unstable and pose a danger to those on board. An improperly throttled or trimmed boat will consume excessive amounts of fuel.